1. Field of the Invention
The present invention relates to a single-sideband (referred to as SSB hereinafter modulator. More particularly, the present invention relates to a digital SSB modulator capable of adjusting the carrier amplitude level of the modulated SSB signal.
2. Description the Related Art
It is known that one analog SSB modulation method with an adjustable carrier level shifts the frequency of the carrier to an passband of an SSB filter.
However, in this case, since the unwanted sideband overlaps with the passband of the SSB filter, the suppression of the unwanted sideband becomes poor, especially in a low-frequency range. Another method for obtaining an adjustable carrier level is adding a fixed carrier to a suppressed carrier SSB signal.
However, it is very complicated to adjust the carrier frequency and the carrier level in this case, and the carrier frequency and the carrier level may change following the passage of time. The disadvantages of the analog SSB modulation methods noted above can almost be overcome by using digital signal processing. In the digital processing, in order to increase operation accuracy, the sampling frequency is set as high as desired.
However, in this case there isn't have enough time to finish the complicated arithmetic operation. For solving this problem, multi-rate processing is usually used to reduce the sampling frequency to 1/L by decimeter 101, as shown in FIG. 13, where L is a positive number.
In the digital modulator shown in FIG. 13, at first, a digital signal is produced by sampling an analog signal at a sampling frequency fs.
The digital signal is filtered by an LPF and is decimated by L in the decimeter 101 and then, input to a phase shift network 102. Two signals which have a 90-degree phase difference are produced by the phase shift network (referred to as a PSN hereinafter) 102 and interpolated in an interpolator 103.
Then, the two obtained signals are input to two LPFs and the output signals of the two LPFs are respectively multiplied by carrier signals which are generated by carrier signal generators 104 and 105. The two obtained signals are added in an adder 106 to produce a suppressed carrier SSB signal. A carrier whose level can be adjusted is added to the SSB signal by an adder 107 and then, the SSB signal is converted from a digital signal to an analog signal so that a low carrier level SSB signal, a full carrier level SSB signal or a RZ-SSB signal can be obtained.
However, if the sampling frequency is set as high as desired for improving the operation accuracy in the digital processing, the operation efficiency in the digital signal processor becomes poor since at least two sampling frequencies fs and fs/L are involved in the digital signal processing which are brought about by the decimation and interpolation.
Furthermore, the modulation characteristics are affected by the quantum error (round off error) of the LPFs which are used in the decimation and interpolation.